1. Field of the Invention
The present invention relates to a video signal scramble system for use in a subscriber television system such as a cable television (CATV) system.
2. Description of the Related Art
To prevent non-subscribers from tapping a TV program, such a CATV system is designed to scramble a video signal of a paid TV program before transmitting it so that only those subscribers who have key data to descramble the signal can watch, and enjoy, the TV program. There are various scrambling methods developed for this purpose. A sync compression method is one of them and is relatively easy to realize. According to this method, a sync signal component of the radio frequency (RF) or intermediate frequency (IF) video signal to be telecast is subjected to level compression before the video signal is transmitted. A control signal for indicating the timing at which the level compression is executed, is superposed on an audio FM signal in AM modulation before it is transmitted to subscribers. On a receiver side, the control signal is extracted, an expansion pulse is prepared based on the timing of this control signal, and the level-compressed component of the scrambled video signal is expanded on the basis of this expansion pulse. Through the above procedures, a demodulated video signal can properly be displayed.
When the transmission timing of the control signal coincides with its horizontal sync interval, program tapping is relatively easy. To prevent this tapping, therefore, there is a method proposed which transmits the control signal at a timing not matching the horizontal sync interval (or with a delay), as is disclosed in U.S. Pat. No. 4,466,017. With the use of this method, the program tapping can be made difficult by varying the delay from time to time.
According to a scramble system employing the above methods, since, on the receiver side, an expansion pulse synchronized with the sync compression position is prepared using the control signal reproduced, the expansion pulse would be out of phase to thereby adversely influence a video signal unless the control signal is accurately detected. For accurate detection, the control signal should better be a rectangular wave. When a rectangular wave is superposed in the form of an AM modulation on an audio FM signal, however, the resultant signal has a broader spectrum, which interferes with the video signal. More specifically, the spectrum of the audio signal may be mixed in a high region of the video signal, particularly, in the region of a color signal. In such a case, a significant beat interference with color would occur which appears directly on a TV screen. In the case where the control signal is given in the horizontal blanking period, a color burst is interfered with, resulting in color noise.
Further, if the rectangular wave control signal is superposed on an audio FM signal in the form of an AM modulation, the audio signal may also be adversely influence. That is, in the above case, due to the broader spectrum of the resultant signal, a spectrum not inherent to a sound carrier appears around it. Since there is always one control signal for one horizontal interval according to the above scramble system, spectrums of 1fH, 2fH, 3fH and so forth appear. For a stereo audio signal, a stereo subcarrier exists at the 2fH position and a stereo/multiplex switching signal exists at the 3.5fH position, so that the spectrum of the control signal is mixed in these signals as noise.
In addition, according to the above conventional methods, if an AM impulse noise is mixed in, for example, a sound carrier, it may be mistakenly extracted as a control signal.